1. Field of the Invention
This invention relates to a process for oxidizing a metal of variable valence by electrochemical means. More particularly, this invention relates to a process for oxidizing either or both vanadium and uranium contained in wet process phosphoric acid to a higher valence state for extraction by subsequent contact with complex organic extractants.
2. Description of the Prior Art
Various commercial processes require a metal of variable valence in an aqueous or organic solution to be in a particular valence state. For example, there are processes for the simultaneous coextraction of pentavalent vanadium and hexavalent uranium from aqueous acidic solutions containing the same, such as, for example, the process disclosed in U.S. Pat. No. 3,836,476. There also are numerous processes for the recovery of vanadium or uranium separately from wet process phosphoric acid.
Wet process phosphoric acid is produced by contacting phosphate rock with a mineral acid such as sulfuric acid. Most phosphate rock contains metal compounds in varying amounts. In many cases, these metal compounds are dissolved from the phosphate rock and appear in the wet process acid as contaminants. Vanadium and uranium compounds are among those dissolved from the phosphate rock, particularly when the rock is from the so-called western deposits of Idaho, Wyoming, Utah and Montana.
In the coextraction processes and many of the other processes, it is necessary to oxidize the vanadium and uranium in the wet process phosphoric acid to a higher valence state to enable the vanadium and uranium to be separated.
In the past, this oxidation has been achieved through the addition of chemical oxidants in an amount in excess of the stoichiometric requirement for oxidizing all the vanadium to pentavalent vanadium and all the uranium to hexavalent uranium. Normally, the oxidant is added in an amount of from about 50 percent to 1000 percent in excess of that which is stoichiometrically required. Various oxidizing agents have been used such as chlorates, manganese dioxide, permanganates, dichromates, peroxydisulfates, and ceric salts.
The disadvantages resulting from chemical oxidation are increased process expense, the possibility of increased plant corrosion, and further contamination of the acidic solution through addition of the oxidizing agent.
Though while not applied to the oxidation of vanadium and uranium, it is known that phosphoric acid containing trivalent iron impurities can be purified by direct current electrolytic reduction of the iron to the divalent oxidation state and precipitation of the iron impurities by the process disclosed in U.S. Pat. No. 2,288,752, or by direct current electrolytic reduction of the iron impurities and recovery of the phosphoric acid by extraction with a water-insoluble amine extractant, such as disclosed in U.S. Pat. No. 3,479,139.
The principal disadvantage of these reduction processes is that they operate by passing a constant direct current through the phosphoric acid electrolyte to purify the material. Thus, the prior processes do not provide a means of controlling the electrolysis, resulting in decreased current efficiency and thereby increased operating expenses.
It is desirable to provide a process whereby a metal of variable valence, such as, vanadium or uranium may be oxidized automatically by electrolytic means with improved current efficiency.